The present invention generally relates to techniques for measuring the thickness of thin insulating layers and associated oxide-silicon interface states charge density and, more particularly, to such measurements of very thin oxide layers of the order of 200 Angstroms and less and to corona discharge guns for use therewith.
Measuring the thickness of relatively thin oxides, DRAM storage node oxides, etc., normally can be done readily by optical or electrical instrumentation. For example, U.S. Pat. No. 5,343,293 issued to Rudolf Berger, et al. on Aug. 30, 1994, and the references cited therein, disclose optical ellipsometer means for measuring the thickness of oxide films on silicon wafers based upon a discernable change in polarized light passing through the film. The discernability or quantification of said change decreases sharply for reduced thicknesses of oxide films below about 200 Angstroms.
U.S. Pat. No. 4,780,680 issued to Klaus Reuter, et al. on Oct. 25, 1988 teaches a contactless electrical technique suitable for making approximate thickness measurements of relatively thick strips of insulating materials and insulating coatings such as tape and paint. Approximate oxide thickness measurements might be possible if the cited electrical technique were practiced on oxide layers on silicon. However, for reasons not explained in the patent but fully described hereinafter, the cited technique could not yield useful thickness results for oxide layers below about 200 Angstroms.